Functional twin boundaries: Multiferroicity in confined spaces

Device materials where the functionality is based on the internal twin structures may need high twin boundary densities. Multiferroic materials, martensite wires, and ferroelastic materials have exactly the required properties because their twin density can be changed by strain, electric or magnetic fields. The thermal conductivity of nanomaterials was found to depend sensitively on the number of twin boundaries with an orientation perpendicular to the heat flow. Microstructural patterns of twin boundaries and tweed in ferroelastic materials display typical aspects of glasses such as weak ergodicity breaking. In case of transformation twins, the order parameter is the strain and the phase transitions in the case of mobile twin boundaries under external stress are ferroelastic. In case of CaTiO₃, an extensive Landau analysis shows the intrinsic instabilities to form polar twin boundaries. A unified description is provided by Landau theory of coupled-order parameters and the emerging property of chiral twin walls.